(1) Field of the Invention
The invention relates to a new type of electron capture detector (ECD) for measuring the concentration of trace sample materials in a carrier gas stream such as the eluent of a gas chromatograph. An important improvement provided by this invention is that smaller trace sample concentrations are detectable, that is, the ultimate sensitivity is substantially improved. Also an improved dynamic range of quasi-linear response to sample concentration results, and there is an improved stability against drift and aging. Furthermore, with this invention the applicability of ECD-type devices is extended to classes of chemical species which form positive ions but not negative ions and thus are not detected in conventional electron capture detectors.
(2) Discussion of the Prior Art
Electron capture detectors (ECD) are employed as highly sensitive detectors for trace quantities of molecular species having large reaction rates for forming negative ions in an inert gas plasma at about atmospheric pressure. Many species of environmental interest, such as pesticide residues, fall into this class. The ECD is typically employed to measure the concentration as a function of time of such species in a flowing carrier gas stream. Most often the carrier gas stream is the eluent from a gas chromatograph column.
Essential features of ECDs in conventional prior art embodiments include a well defined and temperature controlled reaction volume through which the sample bearing carrier gas flows at a controlled rate, a source of ionization within this volume (for example a quantity of appropriately confined .sup.63 Ni or .sup.3 H radiating .beta.-rays which produce the required carrier gas plasma), and a pair of electrodes (generally delimiting the active volume) for the purpose of collecting mobile charged particles from the active volume when an appropriate voltage is applied between the electrodes. Auxiliary apparatus includes an instrument to measure the current flowing in response to the applied voltage, and a source of the collection voltage, which may be either dc or pulsed.
The measured current when electron capturing species are absent is called the standing current. In the prior art, response to an electron capturing sample concentration is observed as a decrease in the current to a value smaller than the standing current. In a variant technique, the collection voltage is applied intermittently as pulses of fixed amplitude and duration, repeating at the feedback controlled frequency required to collect a preselected constant average current. In this mode of operation, increases in pulse frequency rather than decreases in current are measured. For simplicity, whenever changes in current are referred to in the description which follows, it should be appreciated that alternatively, corresponding changes in pulse repetition frequency when the ECD is used in this variant form, may be involved.
The current observed in the presence of sample molecules is smaller than in the absence of same because the sample molecules capture electrons, forming negative ions. The reason that electron capture causes a decrease in observed current is not well understood; a popular explanation is that since the negative ions have a smaller mobility than the electrons they are collected more slowly than the electrons and so have greater opportunity than electrons to be destroyed by recombination with positive ions.
A major problem with conventional prior ECD embodiments is that the response to sample is observed as a relatively small decrease in current superimposed on a much larger standing current. The smallest detectable sample concentration can thus be no smaller than that determined by the requirement that the response exceed the inherent noise level of the device, such inherent noise level being proportional to the square root of the standing current. Thus, if the dynamic range of the EDC is increased by, for example, increasing the strength of the radioactive source, the electron concentration is increased and so is the absolute noise level. It has occurred to the inventor that a preferable mode of operation would be one in which some other parameter is monitored, this parameter having value zero (rather than a maximum) at zero sample concentration, and such parameter increasing (rather than decreasing) in proportion to increasing sample concentration. That is, it is considered that a preferable mode of operation requires measuring a characteristic of the device which has a derivative with respect to sample concentration that is a positive constant, and which has a value of at zero sample concentration of zero. These preferred characteristics are provided by the invention disclosed herein, in which the negative ion density is directly measured.
Another problem occurring with conventional embodiments is that they provide a measure only of the total quantity of electron capturing sample molecules present and do not otherwise distinguish these molecules. Several embodiments of the present invention provide mass spectrometric identification of the electron capturing species. Furthermore, in such embodiments employing mass spectrometric identification, the nature of the positive ions is easily examined, thus extending the applicability of the technique to non-electron capturing species.